Dimethyl ether (hereinafter also referred to as “DME”) is used in many fields in industrial production and by the private consumer. Examples are the use as propellant, e.g. for hair spray, or as starting material for chemical syntheses, e.g. for the preparation of dimethyl sulfate or light olefins (ethylene, propylene, butenes). In addition, DME is a low-emission fuel which is used as an alternative to liquefied petroleum gas from crude oil (“LPG”) and can replace the latter in the long term. The use as low-emission fuel for diesel vehicles has also been tested successfully in a number of countries. DME is usually prepared from synthesis gas (H2 and CO) obtained by reforming of natural gas or by gasification of coal or solids.
The preparation of dimethyl ether is then effected either by direct synthesis from synthesis gas or in two stages via the synthesis of methanol and subsequent conversion of the methanol into DME and water. The DME produced worldwide today is prepared virtually exclusively from methanol. The second stage of this “indirect” DME synthesis, viz. the preparation of DME from methanol, is based on the known reaction design basis of conversion of methanol into DME and water in the gas phase over an acid catalyst, for example over Al2O3, in a single-stage fixed-bed reactor. The following chemical reaction takes place here:2CH3OHCH3OCH3+H2O, ΔH=−24 kJ/mol
The heat of the exothermic reaction is either removed by cooling in the reactor or the gaseous feed methanol is, in the case of adiabatic operation of the reaction, superheated by the heat of the reaction product in a feed heat exchanger. In the case of a cooled reactor, this is typically designed as a tube reactor, with the chemical reaction taking place in the catalyst-filled tubes and the reaction at the same time being cooled by the gaseous feed methanol which is conveyed to the shell side of the reactor and is further preheated there by the heat of reaction.
The version of the methanol-based DME process which is described below as “prior art DME process” is based on the use of a DME reactor. The DME reactor is usually followed by a product work-up using two rectification columns, an DME column and a methanol column for separating off unreacted feed methanol from water, and also an offgas scrubber. This DME process is shown in FIG. 1.
The prior art DME process usually comprises a complicated heat integration, with the hot reaction product being utilized for heating the feed methanol and for operating boilers or for heating streams which are circulated by pumping in the vicinity of the bottom of one of the columns.
There is a continual search for improving the process economics of industrial processes. Possible improvements can relate to the energy efficiency, low purity requirements of the starting materials, higher product purity, productivity and/or the apparatuses used.
The earlier DE 10 2011 114 228 A1, which is not a prior publication, discloses a cooled reactor for preparing dimethyl ether from methanol by heterogeneously catalyzed dehydration. A reactor in which adiabatic heating by means of the heat of reaction liberated in the start zone is firstly carried out, by which means the reaction rate is increased to industrially acceptable values, is used. One of the reactor designs presented comprises a plurality of catalyst beds connected in series. The work-up of the reaction product dimethyl ether is not disclosed.
US 2009/0023958 A1 discloses a process for preparing dimethyl ether from methanol in an adiabatically operated reactor in which two catalyst beds are arranged in series. The process is characterized by the use of selected catalysts in the catalyst beds.
U.S. Pat. No. 4,560,87 A discloses a further process for preparing dimethyl ether and also working up the product obtained. The resulting dimethyl ether is formed in good yield and is obtained in high purity.
It is an object of the present invention to provide an improved process and a plant suitable for this purpose for preparing dimethyl ether, which give a high productivity.
The DME synthesis is an equilibrium reaction. It is independent of or only insignificantly dependent on the pressure. The equilibrium can be shifted in the direction of DME formation by a low working temperature. However, the kinetics of the catalytic reaction at the same time require a minimum working temperature for the chemical reaction to light-off and proceed in a stable manner.
To achieve a high conversion of the equilibrium reaction, it is thus advantageous to work at the lowest possible reactor temperature, which results in a relatively low reactor outlet temperature.
Small DME plants, e.g. for preparing pure DME as propellant, mostly have one cooled reactor. Such plants usually have capacities of from 10 000 to 40 000 metric tons per year. The cooled reactor, designed as a tube reactor with cooling by methanol vapor on the shell of the apparatus, is economically feasible at small to medium plant capacities.
Larger DME plants for producing fuel-grade DME as LPG or diesel substitute usually have capacities of more than 100 000 metric tons per year. The design of such large plants has been known for about ten years, while small plants for producing pure DME have been built for over thirty years. At the construction scale of fuel-grade DME plants, tube reactors are very expensive because of the large number of tubes and because two tube reactors have to be provided in parallel at the largest capacities. Industrially, an adiabatic fixed-bed reactor, which can have, for example, the form of a shaft reactor, is therefore used at such large plant capacities for reasons of lower capital costs.
A disadvantage of the adiabatic mode of operation is that the temperature of the reaction mixture typically increases by more than 100° C. within the reactor. This shifts the reaction equilibrium to a lower methanol conversion compared to a cooled reactor which has a lower outlet temperature. As a result, more unreacted methanol has to be recovered in the methanol column, which significantly increases the capital costs and the operating media costs of this column.